CKD stands for chronic kidney disease, a condition where your kidneys are damaged or functioning below normal for longer than three months. The formal definition, established by the international guideline group KDIGO in 2012, describes CKD as abnormalities of kidney structure or function, present for more than three months, with implications for health. A kidney filtration rate below 60 mL/min (compared to a normal rate of 90 or above) is enough on its own to confirm the diagnosis if it persists beyond that three-month window.
CKD is one of the most common chronic conditions worldwide, and understanding the medical terminology around it helps make sense of lab results, doctor conversations, and treatment plans.
How Doctors Measure Kidney Function
The central measurement in CKD is something called the estimated glomerular filtration rate, or eGFR. This number reflects how efficiently your kidneys filter waste from your blood each minute. It’s calculated from a routine blood test that measures creatinine, a waste product from muscle metabolism. A healthy eGFR is 90 or higher.
The second key marker is the albumin-to-creatinine ratio (ACR), measured from a urine sample. Albumin is a protein that healthy kidneys keep in your blood. When it leaks into urine, it signals kidney damage. Doctors classify albumin leakage into three categories: A1 (normal, minimal leakage), A2 (moderately increased), and A3 (severely increased). You can have kidney damage with protein in your urine even when your eGFR still looks normal, which is why both numbers matter for diagnosis.
The Five Stages of CKD
CKD is classified into stages based on eGFR, giving doctors and patients a shorthand for how much kidney function remains:
- Stage G1 (eGFR 90 or above): Normal filtration rate, but other signs of kidney damage are present, such as protein in the urine or structural abnormalities on imaging.
- Stage G2 (eGFR 60 to 89): Mildly decreased function, again requiring additional evidence of damage to qualify as CKD.
- Stage G3a (eGFR 45 to 59): Mild to moderate decrease.
- Stage G3b (eGFR 30 to 44): Moderate to severe decrease.
- Stage G4 (eGFR 15 to 29): Severely decreased function.
- Stage G5 (eGFR below 15): Kidney failure.
An important detail: stages G1 and G2 alone do not count as CKD. You need separate evidence of kidney damage, like abnormal urine protein levels or visible structural problems, to meet the criteria at those filtration levels. Once eGFR drops below 60 and stays there for three months, the low filtration rate itself is enough for a CKD diagnosis regardless of other markers.
What Causes CKD
Type 2 diabetes is the single largest driver, accounting for an estimated 30% to 50% of all CKD cases and end-stage kidney disease. Persistently high blood sugar damages the tiny blood vessels inside the kidneys that do the filtering work. High blood pressure is the second most common cause, as elevated pressure strains those same delicate vessels over time.
Other causes include autoimmune diseases like lupus, inherited conditions like polycystic kidney disease, recurrent kidney infections, prolonged obstruction of the urinary tract, and long-term use of certain medications that are hard on the kidneys. In many cases, diabetes and high blood pressure occur together, compounding the risk.
Why It’s Usually Silent Until Late Stages
One of the most important things to understand about CKD is that you can lose a significant amount of kidney function without feeling any different. Most people have no symptoms until the disease is advanced. Your kidneys have substantial reserve capacity. Even when half the filtering units are gone, the remaining ones compensate by working harder, which keeps blood test results looking relatively stable for a while.
Symptoms typically emerge in stages 4 and 5, when the kidneys can no longer keep up. At that point, waste products and fluid accumulate in the body, causing fatigue, nausea, loss of appetite, trouble concentrating, and swelling in the legs. Fluid can build up in the lungs, leading to shortness of breath. Blood pressure becomes harder to control. Potassium levels can rise dangerously, posing a risk to the heart. Some people experience personality changes or difficulty thinking clearly as toxins affect the nervous system. Because early CKD is invisible, routine blood and urine tests are the only reliable way to catch it before damage becomes severe.
How Kidney Damage Progresses
CKD tends to get worse over time because of a self-reinforcing cycle. When some of the kidney’s filtering units (called nephrons) are destroyed by disease, the surviving nephrons pick up the slack by filtering more blood at higher pressure. This extra workload causes physical strain on the walls of those tiny blood vessels, triggering the production of scar tissue. Scarring destroys more nephrons, which forces the survivors to work even harder, creating a feedback loop of damage.
The overworked nephrons also demand more oxygen than the surrounding tissue can deliver. This oxygen shortage shifts cells toward less efficient metabolism, generates harmful reactive molecules, and fuels inflammation. Over time, healthy kidney tissue is gradually replaced by fibrous scar tissue that cannot filter blood. This is why CKD management focuses heavily on reducing the strain on remaining nephrons, primarily by controlling blood pressure and blood sugar.
Complications Beyond the Kidneys
As kidney function declines, the effects ripple through other organ systems. One of the earlier complications is a disruption in mineral and bone metabolism, known medically as CKD-MBD (mineral and bone disorder). Damaged kidneys lose the ability to activate vitamin D, which your body needs to absorb calcium from food. At the same time, they struggle to clear phosphate from the blood. The body responds by producing more parathyroid hormone to try to correct the imbalance, but this eventually weakens bones and promotes calcium deposits in blood vessel walls.
This vascular calcification is a major reason why heart disease is the leading cause of death in people with CKD, not kidney failure itself. The mineral imbalances start surprisingly early. A hormone called FGF23, released by bone cells in response to rising phosphate levels, increases even in early-stage CKD, well before standard blood tests for calcium or phosphate look abnormal. FGF23 suppresses vitamin D activation and has direct effects on the heart, linking kidney disease to cardiovascular risk from the start.
Anemia is another common complication. Healthy kidneys produce a hormone that signals the bone marrow to make red blood cells. As kidney tissue is replaced by scar tissue, production of this hormone drops, leading to fatigue, weakness, and shortness of breath that compounds the symptoms of kidney disease itself.
What CKD Means for Daily Life
Living with a CKD diagnosis depends heavily on the stage. In stages 1 through 3, the focus is on slowing progression. That means managing blood pressure, keeping blood sugar in a healthy range if you have diabetes, adjusting your diet to reduce strain on the kidneys (particularly limiting sodium and, in later stages, potassium and phosphate), and avoiding medications that can accelerate kidney damage.
In stages 4 and 5, the conversation shifts toward preparing for possible dialysis or kidney transplant. Stage 5, formally called kidney failure, means the kidneys can no longer sustain life on their own. At this point, dialysis takes over the filtering work, or a transplant provides a functioning kidney. The pace of progression varies enormously. Some people remain stable at stage 3 for decades, while others progress more quickly depending on the underlying cause, how well risk factors are controlled, and individual biology.
Regular monitoring through blood and urine tests is the backbone of CKD management at every stage. Tracking your eGFR and urine protein levels over time gives the clearest picture of whether the disease is stable or advancing, and whether current treatments are working.